Clownfish are probably the easiest marine fish to breed and successfully rear to adulthood.

The same principles should apply to most Clowns and Damselfish who do not scatter their eggs. If you succeed in rearing other species of this Family, I would be most interested in learning from your experience.

Sexing the fish:

Actually, that's the easiest part--just take any two fish, and give them enough time….. Clownfish are all born as males, believe it or not. Then, the largest (and most dominant) of any group undergoes a sex change, and becomes the female. The second largest fish usually becomes the breeding male, and all the other fish remain "sexless" drones. Should the breeding female die or be removed, the breeding male will change to a female, and the next fish in the pecking order will become the breeding male.

So, given enough time any two fish could become a pair, if they're agreeable. It does help to start off with young fish, though. Also, do try to buy your fish from different sources, if possible, to minimize inbreeding.

Breeding tank:

Clowns will spawn in a community tank if they feel secure, and if the water parameters, temperature etc. is to their liking. Try not to have the tank densely stocked, though. If you could keep the breeding pair in a tank of their own it would be even better. They don't need a very large tank, ~200 liters (55 US gal.) is quite sufficient. My tank's water temperature was between 79°F - 82°F (26°C - 28°C), and the nitrates was undetectable. Lighting is not critical, but day and night cycles should be regular.

Spawning:

Some live rock, or other hard, rocky substance with a vertical face is needed for the spawning site. My fish spawned on a rock which was very close to their anemone. All subsequent spawning were on the very same rock. I would recommend that you leave their chosen rock undisturbed after their initial spawning.
When the fish are ready to spawn (within a few days), they will start to clean their chosen rock by vigorously biting it. They also become very aggressive, and will attack other inhabitants. The actual spawning takes place in the afternoon, or early evening, and can last for an hour, or even more. The female swims very slowly over the cleaned area, depositing the eggs. The male then follows close behind, and fertilize the eggs.

Once the spawning is complete (within 1-2 hours), the male assumes responsibility for attending them, while the female acts as protector of the eggs and "supervise" her male. He will continuously fan the eggs with his fins, and even bite at them - not to eat them, but to remove detritus, or perhaps a dead or rotten egg. The eggs should be left in their care, and not removed, unless they are known to be egg-eaters from prior experience.

Hatching:

According to the experts, the eggs take from 6 to 15 days to hatch, depending on the temperature. My eggs hatched as regular as clockwork on the evening of the seventh day. During the last day, the eggs change from a reddish brown to a silvery colour. This is a positive indication that the eggs are due to hatch that evening.

All the power heads and external filters should be stopped the evening of the hatching. Just before the last light goes out, one should also stop the sump return pump. After lights-out, one should check on the tank every 15 minutes or so, using a red filter in front of a torch. About 1-2 hours after total darkness, the eggs hatch, all within a few minutes of each other. At this stage, one should switch on a dim exterior light, just to make it easier to work in the tank. Then, use a bright torch (without filter), and shine the light into the tank at a place convenient to catching the fry. All the hatched fry will immediately start to swim towards the light, and congregate in great masses just below the water surface. It's then a simple thing to scoop them up into a shallow bowl (or even a large soup ladle). The fry should then be transferred to the rearing tank, and gently released by immersing the bowl below the water surface. Care should be taken to ensure as little current and turbulence as possible while doing this, as the newly hatched fry are very delicate.

The first few days:

In nature, the fry would swim to a depth of a several meters below the surface. Being confined to a much shallower tank, they still try to swim away from the light, and will end up "standing" on their heads on the bottom of the tank.. This stress will cause them to die within the first day or two. The rearing tank should thus be kept in total darkness for the first 24-36 hours. Thereafter, the light intensity should be GRADUALLY increased over a period of four to six days, ending with the full power of one 20-30W NO fluorescent lamp. This can be achieved by covering the tank's top with a cover glass, on which one places two portions of dark cardboard. After the initial 36 hour's darkness, one can move the cardboard sections slightly away from each other, thus increasing the "light gap". During this period, the fry should remain free-swimming in the center of the tank. Any tendency to "head stand" should be enough indication that the light intensity is too high.

The Rearing Tank

The rearing tank should ideally be a 7-10 gallon (20-40 liter) tank. The bottom and sides should be painted a dull black, or covered with tight fitting matt black cardboard sheets. (In nature, the light only shines from above, and the fry orientate themselves accordingly. Even the slightest bit of light shining through a side panel will cause the fry to cluster around it, trying to swim through the glass. In doing this, they miss out on food, and invariably die). There should be no substrate, and the heater's pilot light should also be covered (or painted over), else the fry will cluster around the heater..

There should be no filtration in the tank until the fry are at least 3 weeks old. Water movement in the tank should be accomplished by one or two airlines, discharging air from the bottom. During the first 3-4 days air should be pumped through the lines without using an air stone, as the small bubbles tend to "capture" the fry, and cause them to float on the water's surface. This is a sure way to kill them.

Just allow VERY GENTLE aeration, like one or two bubbles per second !!

Water quality, and Filtration

To minimize the growth of bacteria, the rearing tank should be half filled with water from the tank containing the eggs only a few hours before the eggs hatch (the introduction of the fry usually fills the tank completely, with water from the transfer bowl). Of course, the water temperature in both tanks should be identical at the time of transfer.

After 24 hours, one should siphon off about 10% of the water, and replace it with "fresh" water from the main tank. Thereafter, do 10% partial water changes every 12 hours for the first 12 days, and thereafter every 24 hours. During this water change, one should "vacuum" the tank's bottom to suck up any detritus.

Feeding the babies

During the first 24-36 hours the fry do not eat, but live off their yolk sacs. After this time, they should be fed Rotifers three to four times per day. The initial rotifer density should be quite high--it's been recommended to have one rotifer every 1.5 times the length of the fry (meaning that the fry would only have to swim 1.5 body lengths before finding food. After 8-10 days one should start to feed newly hatched Brine Shrimp, and feed this for the next two weeks, while slowly introducing pulverized flake food. After that, you only need to find good homes for your babies.

Cultivating the baby food:

Your first brood of clownfish will probably catch you unprepared, and the chances are good that all the babies will starve within the first 2-3 days. Fortunately, Clowns are prolific breeders, and you can prepare for a successful second batch by starting to cultivate Rotifers, (and the Green-water food for the Rotifers) as soon as the first spawning becomes imminent.

Although Rotifer starter colonies are available commercially in the USA, this is not the case in the rest of the World. I was fortunate to obtain some from the research aquarium of the department of Sea Fisheries, in Cape Town. You will probably find that all universities and aquatic research institutes breed Rotifers, and will be glad to give you some.

Rotifers eat single celled algae, such as Nannochloropsis, Tetraselmis and Chlorella, and without a sufficient supply, you won't have any Rotifers, and that means your babies will starve. Fortunately, starter cultures of these algae can also be obtained from the same source as the Rotifers.

Baby Brine Shrimp (Artemia sp.) culturing should start two days after the babies have hatched. This should give you your first baby Artemia after ~48 hours, just in time for the baby Clowns' gradual weaning to this food type, which should start on the fifth day after hatching.

The following links should take you to sites with much more information than I can give on this subject - I regularly make use of this information myself…